The present invention generally relates to tuning of antennas and, more particularly, to a magnetic amplifier switch for automatic tuning of a very low frequency (VLF) transmitting antenna.
High power VLF transmitters require large antenna systems to obtain any reasonable degree of radiation. Generally, antennas for transmitters are at least one quarter wavelength in dimension. Since the VLF band is 15 kHz to 30 kHz, a quarter wavelength would be 5000 to 2500 meters. Antennas of such great lengths (which also must be an equal distance from the ground plane) are not practical to build.
Therefore, antennas for VLF transmitters are made physically smaller and are reactively loaded to resonate at the desired frequency. The ratio of the reactive power to the real power driven into the antenna circuit (which has been tuned by reactance) is referred to as the "Q" factor. Usually the "Q" factor is greater than 250. In order to produce signal radiation at an acceptable efficiency, tuning the antenna to the exact transmitter frequency is desirable. Any error in tuning will cause a decrease in transmitted power and a reduction in transitter efficiency, i.e., a decrease in the transmitted signal reception area.
Currently, in a VLF transmitting station the antenna is tuned to a desired center frequency. However, the transmitter is typically being operated to frequency modulate the radio frequency power exciting the antenna. This mode of transmitter operation causes a frequency error above and below the center frequency of the tuned antenna, resulting in lower transmitted power. Since VLF transmission is at very high power, typically more than a million watts, the lost power due to the frequency error is quite significant. The frequency modulation (called "mark" and "space" frequencies) is approximately one percent of the center (tuned) frequency. With an antenna "Q" factor of around 250, it can be shown that the power loss due to out-of-tuning can be as much as 4 decibels, or more than half of the power.
In view of the above-described drawback of the current VLF antenna tuning approach, there is a need for a different approach to reduce power loss and increase transmitter efficiency.